Title

The development and evolution of the abdominal gills of mayflies

Date of Completion

January 2007

Keywords

Biology, Molecular|Biology, Entomology

Degree

Ph.D.

Abstract

Mayflies (Ephemeroptera) have highly variable abdominal gills. Gill diversity has historically been linked to the impressive variety of aquatic habitats colonized by nymphal mayflies. I used molecular phylogenetics, ancestral state reconstructions and correlated character analyses to explore evolutionary transformations in gill morphology in leptophlebiid mayflies. Additionally, I used gene expression assays in mayfly embryos and SEM imaging of nymphs to chronicle the molecular and morphological development of Ephoron leukon Williamson (Polymitarcyiidae). ^ In Chapter 1, I present the first phylogenetic hypothesis for Leptophlebiidae using two nuclear genes (28S ribosomal DNA and histone (H3)) and taxa from a broad geographic region. Phylogenetic analyses revealed that the Leptophlebiidae are not monophyletic due to the inclusion of the sampled ephemerellid species. Two of the subfamilies (Leptophlebiidae and Habrophlebiinae) are monophyletic, while Atalophlebiinae is paraphyletic. Several biogeographically sensible subgroups in Atalophlebiinae were consistently resolved. Chapter 2 explores the tempo and mode of gill evolution using ancestral state reconstructions and correlated character analyses. Ancestral state reconstructions uncovered the repeated independent evolution of particular gill morphs and correlated character analyses suggested that the number of basal gill branches and overall gill shape are linked to the relative abundance of oxygen in the aquatic habitat. ^ Chapter 3 documents the first gene expression data for a mayfly species, Ephoron leukon, characterizing the expression of wingless (wg) and the localization of Engrailed (En) over embryonic development. wg and En delimit the body segments and function in nervous system patterning and patterns of expression of wg and En in E. leukon are similar to those described from other insects/arthropods, including iterated stripes on each body segment, across the ventral region of presumptive appendages and in the nervous system. ^ Chapter 4 is an account of early nymphal development in E. leukon using SEM, focusing on mouthpart and gill development. I document the origin of major changes in gill and mouthpart morphology over early instars including the splitting of gills into two lamellae, the appearance of marginal gill fringes and the outgrowth of mandibular tusks. I also describe several ultrastructural details of gills including cuticular discs and putative mechanosensory structures. ^